It is well known that water dispersible polyisocyanates can be used as crosslinking agents for the preparation of two-component water-based laminating adhesives. Such adhesives have been shown to form laminates having enhanced bond strengths when subjected to increased chemical, thermal and mechanical stresses. Generally, the polyisocyanates are admixed with aqueous polyurethane dispersions and coated onto a flexible film substrate. The coated substrate is then dried and mated with a second flexible substrate, using heat and/or pressure, to form a dry-bond laminate. Several references are described below:
U.S. Pat. No. 5,494,960 (Rolando et al.) discloses two-component water-based laminating adhesives comprising water dispersible polyisocyanates and aqueous polyurethane dispersions. The reference describes isocyanurates as undesirable trimerization products (Column 1, lines 49-57).
U.S. Pat. No. 5,250,610 (Hansel et al.) discloses laminating adhesives comprising anionic polyurethane dispersions. The reference also states, "Additives conventionally used in the processing of adhesives may also be added to the dispersions of laminating adhesives according to the invention, e.g., leveling agents, defoamants, viscosity regulators, crosslinking agents, catalysts, etc." (See column 4, lines 28-32.) The reference fails to teach or suggest the use of water dispersible polyisocyanates based on the isocyanurates of hexamethylene diisocyanate.
Other related documents include: U.S. Pat. No. 1,128,568 (Keberle et al.); U.S. Pat. Nos. 3,887,757 and 3,982,986 (Stone et al.); U.S. Pat. No. 4,147,679 (Scriven et al.); U.S. Pat. No. 5,334,690 (Schafheutle et al.); U.S. Pat. Nos. 4,433,095 and 4,663,377 (Holmbach et al.); U.S. Pat. No. 4,540,633 (Kucera et al.); U.S. Pat. Nos. 4,851,459 and 4,883,694 (Ramalingam) and U.S. Pat. No. 5,532,058 (Rolando et al.).
A drawback with the two-component water-based laminating adhesives described above relates to the particle size distribution, prior to the laminating process. The addition of water dispersible polyisocyanates, to aqueous polyurethane dispersions, often increases the adhesives average particle size distribution. The polyisocyanates react with water and/or other active hydrogen atoms on the aqueous polyurethane dispersion (i.e., amines and hydroxyl groups) to form a vast three dimensional network, which tends to destabilize the dispersion. Such a distribution can adversely affect the dispersions coalescing properties to form adhesives having poor machining characteristics, dry-bond clarity and water resistance.
There remains a need for two-component water-based laminating adhesives, based on polyisocyanate crosslinking agents, having enhanced machining characteristics, bond clarity and water resistance.